A short discussion of laser fundamentals is beneficial to an understanding of the prior art. A gas laser generates a monochromatic light beam having a very narrow frequency bandwidth. Thus, the exact wavelength of the light generated by a laser may be accurately determined. The frequency of a generated laser beam will fall somewhere within a finite gain profile of the spectral line bandwidth of the lasing material, which is defined by the gas species and the nature of the spectral broadening mechanism (e.g., Doppler broadening, pressure broadening, etc.)
A Doppler broadened profile of the 633 nm laser transition in helium-neon is graphically shown in FIG. 1, with light frequency plotted relative to intensity for a light beam emitted from excited He-Ne lasing material. The generally bell-shaped curve indicates along the X-axis the range of frequencies or spectral bandwidth within which stimulated light emission can be obtained. The frequency of an He-Ne laser beam will thus fall somewhere within this spectral bandwidth, as indicated by the lines f.sub.1 or f.sub.2, though the exact frequency position depends critically on the instantaneous length of the resonating chamber within which the lasing material is excited.
The Doppler broadened profile of the central spectral line f.sub.o characteristic of He-Ne lasing material typically has a bandwidth .DELTA.f of 1500 MHz. The instantaneous bandwidth of a generated laser beam exists within the Doppler broadened bandwidth, as indicated for example by f.sub.1, and is very narrow in comparison. The frequency of the laser beam may vary within a very large range, comparatvely as one in 10.sup.6, which permits a wide range of light wavelengths to be generated unless means is provided to control the frequency of the laser beam.
The frequency of a laser beam can be controlled by modulating the length of the resonating chamber in which the spectral emission is generated, i.e., the distance between the reflecting surfaces at the ends of the lasing chamber. This can be accomplished by varying the length of the chamber through electromechanical and/or thermal length varying elements. Prior art has instructed use of a piezoelectric element mounted within or on the outside of the casing of the lasing chamber, which changes length when control voltage is applied to provide immediate changes in chamber length. Heating elements or coils have been applied to the exterior of the chamber casing to induce thermally responsive changes in length, which provide a greater range of adjustment while being slower to react.
Through use of these techniques, the narrow bandwidth frequency of the laser beam may be positioned, or tuned, to a desired position within the spectral profile, and stabilized at a desired frequency.
When a gas laser is subjected to an axial magnetic field, circular birefringence induced in the active lasing material by the magnetic field produces a Zeeman-splitting effect on laser beam frequency. The result is the formation of two individual component frequency modes in the laser beam having opposing right and left circular polarizations, differing measurably in frequency. The frequency difference between the component modes is represented as .DELTA.f.sub.z in FIG. 2, which difference may vary on the order of 100 to 1500 KHz depending upon the strength of the magnetic field applied. Component frequency modes split by Zeeman effect are illustrated in FIG. 2 as F.sub.1R and F.sub.1L, F.sub.2e and F.sub.2e, and F.sub.3R and F.sub.3L, which differ in frequency by .DELTA.F.sub.1. The Zeeman effect on laser beam frequency results in a split of the spectral line within the Doppler broadened profile into two components, indicated by the shifted "Doppler" profiles to the right and left of the original "Doppler" profile (indicated by dotted line F.sub.o). Each of the component frequency modes of the laser beam are likely to have a different light intensity, depending upon their position within the Doppler broadened spectral profile. This is indicated in FIG. 2 by the component line F.sub.1R having a greater intensity as measured along the Y-axis, than the component line F.sub.1L. Similar results are visually explainable for F.sub.2 and F.sub.3.
The magnitude of .DELTA.f.sub.1 l depends upon the magnetic field strength and the particular location of the frequency line of the laser beam within the "Doppler" profile. The difference in frequency .DELTA.f reaches a minimum as the component frequency modes become symmetrically positioned about the line center of the original profile (f.sub.0). Thus, minimizing the frequency difference .DELTA.f between component frequency modes provides means of stabilizing the frequency at which the laser is operating. Similarly, intensity difference between the component modes differs depending on the particular location of the frequency line within the "Doppler" profile, while becoming closely equal at some point near the line center of the original profile (f.sub.o), as shown by F.sub.3.
When the component frequency modes of the laser beam are heterodyned, the resultant waveform has a frequency representative of the diffrence in frequencies .DELTA.f between the component frequency modes. The frequency of the heterodyned waveform exhibits a characteristic beat signal, which is well known from the laws of wave physics. The characteristic beat signal obtained from heterodyning the differing frequency modes provides a measure of the difference in the frequencies of the mixed waves. The resultant beat signal is characterized by a first frequency which is an average of the frequencies of the mixed waves, and exhibits an amplitude oscillation, or beat, over time having a much slower second frequency. The second frequency or number of beats occurring per second, is a measure of the difference in frequencies of the combined waves described above. With regard to lasers subjected to a magnetic field to obtain Zeeman splitting of the laser beam frequency, this is referred to as the Zeeman beat signal of the component frequency modes.
The frequency difference between component frequency modes can be easily determined by digitally processing the beat count of the Zeeman beat signal per unit time. Thus the Zeeman beat signal provides a clear indication of frequency difference .DELTA.f between the component frequency modes F.sub.1R and F.sub.L, and can be used to control or stabilize the frequency of the magnetically influenced laser. These properties and phenomena of lasers influenced by a magnetic field have been taught by a number of published studies.
A laser permits the use of a direct measurement method using interferometry in which the wavelength of the laser beam serves as a standard unit of length for measurement. The laser is advantageous as a standard of measure due to its charactristic narrow frequency bandwidth, sharp focus and high intensity light beam, which exhibits an identifiable and accurately measurable light wavelength. The use of a laser in interferometry permits accuracy in length measurement to within fractions of a micron and finds itself to use of sample digital processing techniques to perform length measurement. For measurements of length using interferometric technique, in which the wavelength of the laser beam is used as a standard unit of measure, it is obviously advantageous to stabilize the laser frequency at a predictable and constant value to enable accurate measurement. Preferably, the laser beam should be stabilized with high accuracy of at least one in 10.sup.7.
The use of a laser exhibiting a Zeeman splitting of its beam frequency is particularly advantageous due to its characteristic beat signal, which has a continuous and easily measurable frequency.
Prior art teachings have shown a number of systems for controlling the frequency stability of an output lasr beam. For instance, Lang and Bouwhuis have taught a means of tuning a laser by inducing electrostrictive and thermal changes in the length of the lasing inducing chamber. The lasing chamber length is adjusted to stabilize the median frequency of the laser beam (the median frequency being the average frequency between the Zeeman split component modes) at the known spectral line which is chracteristic of the lasing material. Frequency stabilization is accomplished by measuring the intensity difference between the component modes and using the intensity difference measurement to generate a control signal to correct chamber length. The intensities of each of the component modes are alternately measured by an intensity sensitive photodetector. This is accomplished through selective transmittance of each of the component mode beams through an electro-optical crystal whose birefringence is modulated by an a.c. sgnal to alternately pass one or the other of the component frequencies for intensity measurement by the photodetector. This results in an alternating signal which is compared with the a.c. signal applied to a crystal to obtain a measurement of the difference in intensity of each of the component modes. A control signal is generated responsive to the intensity difference, which controls voltage applied to the chamber length tuning elements of the laser.
The chamber length is adjusted to equalize the intensities of the component modes, thus positioning each of the component modes symmetrically about the line center of the spectral profile, as discussed earlier and observable in FIG. 2. This also obtains a minimum frequency difference .DELTA.f between component modes.
U.S. Pat. No. 3,534,292 of Cutler discloses in a system for modulating the length of the lasing chamber, through use of a piezoelectric element, to produce a frequency difference .DELTA.f between component modes which is continually modulated. A signal representing the modulated frequency difference .DELTA.f.sub.1-2 is supplied to a frequency discriminator, which converts the signal to one having an a.c. and d.c. component. The a.c. component is used to control the range of modulation of the lasing chamber length, and thus .DELTA.f.sub.1-2, through use of a phase shift circuit. The a.c. component is detected to generate an error correction signal which is applied to the piezoelectric element to stabilize the laser component frequencies about the line center of the spectral profile of the lasing medium. The magnitude of the frequency difference between component modes is controlled by a differential amplifier which references a d.c. reference voltage supplied thereto. The differential amplifier generates a signal to control the strength of the magnetic field applied to the lasing chamber.
Morris and Ferguson have discussed a method of frequency stabilization for a laser influenced by a magnetic field, which consists of heterodyning the component mode frequencies and applying the heterodyned signal to a comparator. The comparator provides a frequency-to-voltage conversion signal which is integrated. The signal received from the integrator determines power to be applied to a heating element wound around the laser cavity wall which introduces a thermal adjustment to the length of the lasing chamber. This system is used to control the position of the component mode frequencies within the spectral profile.
Hall in patent application Ser. No. 300,363 filed September 1981 teaches a method of stabilizing the frequency of a laser which comprises obtaining an error signal by dithering (frequency modulating) the Zeeman split component modes within the Doppler broadened spectral range and measuring the difference in the component mode frequencies caused by each dither. An updown counting technique is used to measure change in frequency responding to each direction of a dither, which measurements are compared to determine equal change. Laser cavity length is adjusted to obtain a minimum frequency change of the component modes throughout the dithering cycle. Obtaining an equal frequency change with each dither centrally positions the component modes symmetrically about the line center of the spectral profile. The laser cavity length is servo controlled to maintain minimum frequency difference between component modes by continually applying the dither and counting the relative change in frequency difference between component modes in each direction of its cycle. The counts are maintained equal and opposite in sign.
The Hewlett-Packard Company, Inc. manufactures a gas laser utilizing the Zeeman splitting effect to obtain two component frequencies, which is identfied as Model 5525A. By a method similar to that of Lang and Bouwhis the laser is tuned to generate a beam having a frequency at the line center of the spectral profile. Timing is performed through control of a piezoelectric wafer which forms part of the wall structure of the laser cavity. The piezoelectric element is controlled by an electronic servo control loop. The control loop separately measures the intensities of each of the component frequency modes and compares the intensities measured to make them equal through adjustment of the length of the laser cavity. Equating the intensities of the component modes centers the component mode frequencies about the line center of the spectral profile. Thus, the frequency of each of the Zeeman component modes is controlled to equally differ from the frequency of the line center of the lasing material spectrum profile and the frequency difference between the components is maintained at a minimum to allow accurate prediction of the frequency difference.
Each of the above-described means and methods of stabilizing the frequencies of the components of a laser beam influenced by Zeeman splitting has failed to provide a highly accurate control of the difference in frequency .DELTA.f between component frequency modes, or in other words, the beat signal exhibited by their combination. Control of the frequency difference is a highly important funtion in an inferometric system which uses the beat signal as a standard unit of measure. The Lang and Bouwhis, and Hewlett-Packard frequency stabilization systems adjust the component mode frequencies to symmetrically flank the line center of the spectral profile, at which point the difference in frequency between the component frequency modes is a minimum and most predictable.
In these control methods it is the frequency of each component which is controlled within the spectral profile of the lasing material to obtain a predictable difference in frequency between them, resulting in a constant beat signal upon their combination. This has been accomplished through a measurement function (of intensity) performed on each of the component frequency modes, thus introducing an additional possibility of error in the control system. These measurements, and related control operations require more complicated electronic circuitry. Since with these control methods the actual difference in frequency is not controlled, the frequency difference may not be accurately controlled and may change from one laser to the next. Furthermore, the value of the difference in frequency may be disturbed by environmental magnetic fields even though the frequencies of component frequency modes are being accurately controlled, resulting in differing beat signal frequencies in differing environments.
Nor is the frequency difference .DELTA.f directly controlled by Hall or Cutler. The method described by Hall seeks to minimize the difference in frequency between component frequency modes. The dither or frequency modulation technique used to obtain an indication of the frequency difference to determine when a minimum value is reached requires complicated and expensive servo electronics. Furthermore, dithers from optimum stability of component mode frequency constantly causes a slight change in frequency difference, which derogatorily affects stability and thus constancy of the beat signal. Also, the necessity of performing a dither reduces frequency response. The continual modulation of the length of the lasing chamber and thus the difference in frequency taught by Cutler clearly affects the ability of the servo system to accurately stabilize the difference in frequency of the component modes. This technique approximates that of Hall in applying a dither to continually change the frequencies to obtain a comparison value indicative of a minimum frequency variation.
The Morris/Ferguson system admittedly has an observed frequency difference variation of 200 Hz which clearly indicates that the ability of the system to control the frequency difference is limited. Furthermore, there is no indication of the reference to which the beat signal is compared to obtain a frequency-to-voltage conversion. It would seem that the teaching presented merely indicates that the frequency difference signal may be integrated to obtain a time based control signal adapted to tune the length of the lasing chamber.
Each of the above-referenced teachings attempts control of the frequency difference between component frequency modes of a laser effected by Zeeman splitting through indirect techniques which position the individual component mode frequencies symmetrically within the Doppler broadened profile of the spectral line of the lasing material, to obtain a predictable value of the difference frequencies. A clear need remains for a control which can accurately and directly determine the frequency difference, or beat signal, produced by the component frequency modes of a laser influenced with a magnetic field. Accuracy in stabilizing the frequency difference is clearly advantageous in interferometric techniques for measurement where the frequency difference or beat signal supplies the basic unit used for measurement.